The present invention relates to alarm monitor systems, and in particular to monitor systems wherein a plurality of remote sensor units communicate in a time multiplexed fashion with a central control unit.
Alarm monitoring systems wherein a central control unit communicates with a plurality of remote sensor units are in general well known. Such prior art alarm monitoring systems, however, generally utilize separate transmission lines between the central control unit and the respective remote sensor units, complex interrogating schemes or both. An example of such systems is described in U.S. Pat. No. 3,697,984, issued Oct. 10, 1972 to Atkinson.
To avoid the necessity of complex interrogation schemes in two-wire bus communication systems, sequential addressing of respective transponders along the bus has been achieved in various systems by generation by the central control unit of a single interrogation pulse to which each of the transponders is responsive and interjecting delays into the bus line between the respective transponders. Examples of such systems are described in U.S. Pat. No. 2,723,309 issued Nov. 8, 1955 to Lair et al and U.S. Pat. No. 3,510,841 issued May 5, 1970 to Lejon.
A communication system utilizing a two-wire bus, wherein a plurality of stations connected to the bus communication with each other in a time multiplex fashion, is described in U.S. Pat. No. 2,406,165 issued Aug. 20, 1946 to Schroeder. A synchronization pulse is generated on the bus line, to which all of the stations are responsive. Each station includes a sequence of multi-vibrators (one shots) which are triggered in response to the synchronization pulse to enable the respective station receivers during different sequential time periods. Communication between stations is effected by generating pulse amplitude modulated pulses during the time period associated with the enabling of the receiver in the desired (called) station.
In a sensor monitoring system it is desirable that the system be open-ended, that is, amenable to addition of further sensor units, and further, that the sensor units require no external source of power. An example of such system is disclosed in U.S. Pat. No. 4,053,714 issued Oct. 11, 1977 to Long. In the Long system a plurality of sensor units communicate with a single receiver over a two-wire bus line in a time multiplexed fashion. Each sensor unit includes a crystal controlled clock, and a rechargeable power source (nickle-cadmium battery). A central power supply connected to the bus line periodically applies a charging pulse of current to recharge the batteries in the respective transmitters. The charging pulse provides a peak pulse battery charging current of about 1.5 amp. over a recharging interval of about one second. The recharging pulse is also utilized to reset the respective clocks of the transmitters. The clocks selectively enable the transmitters during respective periods in a predetermined time sequence, whereupon the transmitter transmits a bipolar pulse code message to the receivers. After transmission, the transmitter is effectively disconnected from the bus line until the clock associated therewith is reset by the next recharging pulse.
It should be appreciated that the relatively high currents of the charging pulse sets a limit on the minimum gauge of the bus line wires. Further, the use of high speed digital pulse codes (on the order of 500 BPS) requires use of either coaxial cable or shielded wire to prevent high frequency radiation and/or distortion.
It is desirable to provide an open-ended two-wire bus line sensor monitor system which can utilize relatively inexpensive wire (e.g., Class II) rather than shielded wire or coaxial cable as required in systems utilizing high bit rate digital pulse coding techniques. Dual-Tone multi-frequency (DTMF) modulation techniques are well known in telephony systems as an alternative to digital pulse techniques and have been suggested for use in computer signalling and control system applications.